Electric Brake

I've had friction and engine braking with the blue area and battery not full, at times. It's not perfect.

 

Yes you got that right. Also the less aggressive you have to brake the less pads will be used. I Like to use B mode to help with abrupt braking like when a light turns red going downhill. Just like I use compression braking on a non hybrid truck I drive for work. my pads last along time.
I hope you enjoy your new car as much as I do mine. It is very interesting and efficient.

 

Although, if you use the brake pedal in D, you'll get more energy into the battery.

 

If you were to try both doing that and not doing that, enough times under controlled conditions, you'd probably see that it isn't necessary and isn't doing what you think. The car has two ways of slowing without using the pads: reclaiming energy electrically, or spinning it off in the engine. Both of those methods avoid wear on the pads, and the car chooses between them normally anyway. B mode only hints to the computer to have a higher preference for engine-spinning over regen, but that's just trading between two methods that equally save your pads.

The other thing B mode does, to make it feel like downshifting in a conventional car, is it sets a somewhat higher level of target forward resistance with the go pedal released, but that's just changing a number in a computer.

-Chap

 

I understand what you are saying and respect your understanding of these cars. I have gained much insight from reading your posts. I have Been to Indiana and enjoyed catfish dinner on the Wabash River. I know how flat it is. I live at 560' elevation and go to sea level to go anywhere locally.I am forced to go from 40 to stop in a very short time frequently. I optimize regen braking and try to put the aqua bar at the dashed line in advance of stopping as much as possible. I was pointing out another use of B that reduced pad use and the engine definitely does that well. I can hear and feel the deceleration during a abrupt stop. As a side note the engine is often spinning to release excess energy around these traffic lights anyway with no fuel used.

 

It sounds as if you are more comfortable going to B early, then only later pressing the pedal, and you are enjoying the early deceleration that you hear and feel because B simulates a "downshift" by increasing the pedal-released target braking force. Sounds like you're not comfortable giving the same target input using the pedal alone, possibly out of concern that touching the pedal will always use friction to some extent.

On the first-gen Prius, that was indeed the case (2004 New Car Features Manual, p. CH-36):



Gen 1 had a relatively simple hydraulic system that resulted in the pads applied to some minimal extent (the 'stick' of the shaded hockey stick shown) for any level of brake pedal force.

Two changes introduced with Gen 2 improved that picture. Making the hydraulics electronically controlled made it possible to keep them inactive during the early pedal stroke, shaving off the top of that hockey stick, including the whole 'stick' part. That's the "ECB effect" in the illustration. The more powerful MG2 shaved off more of the 'blade' of the stick by making more regen possible ("Expanded Regenerative Range" in the illustration).

As you're not driving a Gen 1, you really can go lightly on the pedal early, and accomplish what you're now doing with B. It might not always give you the same satisfying engine-spinny sound (if there's battery capacity, it'll fill that first, but that's usually a good thing). If the battery's already full, it will sound spinny right away, just as if you had shifted to B.

Now, if you're watching your HV battery temps, and you're facing enough of a descent to spike those, B mode gives you a way to keep those down. That's the situation where 'fill battery first' isn't always a good thing, and that's what you're telling the car by a shift into B.

-Chap

 

If you're doing a near emergency stop, hit the damn pedal, it'll be faster than switching to B, and your pads are more important than your life and the lives of your passengers and of those ahead of you. And, if you're stopping THAT hard, you're probably mostly on pads anyway.

 

Ah. I suspect some controlled / instrumented testing would persuade you it isn't doing exactly what you think, but rather trading away regen in favor of engine dissipation.

But there is an observation I'm now curious about, and have never made. I usually have battery current showing on my ScanGauge, so I'm pretty familiar with the typical limits on regen (rarely more than 100 amps, or about 20-some kW, that I've seen), but I haven't compared that to the maximum dissipation possible through engine pumping. That would call for watching MG2 power while decelerating with the battery full; my latest trip through the Blue Ridge would have made that easy.

Looks like it would require a setup capable of logging both MG2 torque and MG2 revs, then postprocessing to multiply the two (an app like Torque might allow defining a 'virtual' PID to be the product of two real ones, and show it in near-real-time).

-Chap

 

Never mind. I am not going to repeat the scenario.

 

You need to use the brake pads periodically to keep them from rusting up. There are several posts concerning brakes immobilized by rust and corrosion.

 

Agree!! - that's why we keep inconsiderate drivers on the road, forcing the brake disks to be used for the occasional panic stop.

 

I think there's always some use of the brake pads, that it's not either/or. Whenever I start out after the car's been washed and subsequently sat a day or two, I can hear the pads scrubbing off surface rust each time I step on the pedal. In a block or two the noise goes away.

 

I don't think that's a likely issue as long as you're driving the car anything like regularly. I don't care how clever you are with managing your regen and B mode, your pads will get used in normal driving, enough to rub the rust off.

Now sometimes, right in the morning after a rainy or damp night, I like to speed the process along by shifting into N for one or two stops ... all it takes to clean up that scrunchy rusted sound, then they're all quiet and smooth again.

-Chap

 

I do not have a OBDII monitoring App yet. I want one. Again I go up and down a lot of hills with traffic lights. For now I have been using the charge line on the display when braking down hills. I was assuming that was the regen capacity is maximum at the dashed line and below that line was friction brakes. The owners manual is not clear about that.If I need more slowing then I have been putting it in B to supplement the regen that I was already trying to maximize. I look forward to knowing coolant and HV battery temperatures in the future so I can better manage the car because I enjoy it. I agree the car takes care of itself pretty well though.

 

AFAIK, regen will actually reduce in B, though - maximum regen is only available in D, based on what others have found.

And, just measuring MG2 won't tell you everything - MG2 power is sent to MG1 to spin the engine when engine braking needs to occur.

 

I think it'd be close enough for a rough answer to my curiosity. The setup would be to get going down one of those long mountain grades until SoC hits 80% and then the engine-braking RPM maxes out, then measure the MG2 power. None of it's going to the battery at that point, and the amount going to the DC converter to run car systems is minor in comparison, so it's pretty much the amount of power that can be collected by MG2 and dissipated by engine pumping.

Strictly speaking, the rate of kinetic energy being burned off is that plus the amount following the mechanical path (which, I think, could be calculated using the MG2 figures, engine RPM, and the nomograph).

-Chap

 

I guess I need to find a longer downhill for a test.

On a 3/4 mile downhill, when I put it in B mode the car barely (if at all) slows down.... and quickly speeds back up. I still have to ride the brakes down the hill.

I'm not concerned about the battery, my main concern is the safety and speed control. I don't feel comfortable in my Prius going down a steep hill. (I would definitely hate it if the roads were snowy or icy.)

 

When you're riding the brakes, does the engine get louder?

The brake pedal isn't the brake pedal, it's the slow down pedal.